Add 'import XMonad.Util.XSelection' to the top of Config.hs
Then make use of getSelection or promptSelection as needed; if
one wanted to run Firefox with the selection as an argument (say,
the selection is an URL you just highlighted), then one could add
to the Config.hs a line like thus:

Returns a String corresponding to the current mouse selection in X; if there is none, an empty string is returned. Note that this is
really only reliable for ASCII text and currently escapes or otherwise mangles more complex UTF-8 characters.

A wrapper around getSelection. Makes it convenient to run a program with the current selection as an argument.
This is convenient for handling URLs, in particular. For example, in your Config.hs you could bind a key to
promptSelection "firefox";
this would allow you to highlight a URL string and then immediately open it up in Firefox.

promptSelection passes strings through the shell; if you do not wish your selected text to be interpreted/mangled
by the shell, use safePromptSelection which will bypass the shell using safeSpawn from Run.hs; see Run.hs for more
details on the advantages/disadvantages of this.